function rhodot=fctode(zm,rho,delta0,lambda0,dn,z)                  
dnz=spline(z,dn,zm);
rhodot=zeros(2,1);
rhodot(1,1)=-2*delta0*rho(2)-pi*dnz*2*rho(1)*rho(2)/(lambda0);
rhodot(2,1)=2*delta0*rho(1)+pi*dnz*(rho(1)^2-rho(2)^2)/(lambda0)+pi/lambda0*dnz;